Link Between Extreme Weather And Climate Change2011 brought exceptionally mild winters in most of the U.S., deadly tornadoes in the Midwest and extended drought in the West and Southwest. Kevin Trenberth, distinguished senior scientist at the National Center for Atmospheric Research, discusses the correlation between climate change and extreme weather.

2011 brought exceptionally mild winters in most of the U.S., deadly tornadoes in the Midwest and extended drought in the West and Southwest. Kevin Trenberth, distinguished senior scientist at the National Center for Atmospheric Research, discusses the correlation between climate change and extreme weather.

NEAL CONAN, HOST:

This is TALK OF THE NATION from NPR News. I'm Neal Conan.

Monstrous tornadoes tore through North Texas earlier this week. You probably saw video of truck trailers tossed around like Frisbees. U.S. tornado season started early this year and has been blamed for 57 deaths so far, none, luckily, in Texas this week. That follows some of the most unusual weather in memory, including an exceptionally mild winter in much of the country and extended drought in the Southwest and parts of California.

If you have questions about whether and climate change, give us a call: 800-989-8255. Email: talk@npr.org. You can also join the conversation on our website. That's at npr.org, click on TALK OF THE NATION. Kevin Trenberth, distinguished senior scientist at the National Center for Atmospheric Research joins us by phone from his office in Boulder, Colorado. Nice to have you with us today.

KEVIN TRENBERTH: Hi, Neal. It's nice to be here.

CONAN: And we're here in the middle of the Cherry Blossom Festival in Washington, D.C. Of course, the cherry blossoms all came and went.

TRENBERTH: They came a bit early today this year.

CONAN: Yes, indeed.

TRENBERTH: Spring has been moving up. It has been generally the case, but - this year especially so. And the tornado season seems to have moved up as well.

CONAN: And that means there are going to be more of these storms, no?

TRENBERTH: Well, you know, tornadoes vary quite a lot from year to year, and they depend on other factors, as to whether the jet stream is in just the right location and certainly the warm moist air coming out of the Gulf. And, you know, that's part of the climate change aspect, is that the air coming out of the Gulf is a bit warmer and moister. And so it helps to invigorate the thunderstorms that host these tornadoes. So that is one factor that's underplayed, but there is a lot of variability, of course, from year to year.

CONAN: Variability from year to year. And that's what a lot of people say. Last winter, we had a lot of snow, for example, in the Northeast. There were scarcely any this year.

TRENBERTH: Yes. Well, that was true. Further west there, you know, you may recall flooding along the Missouri as a result of that. You know, ironically with global warming, you can get more snow in the middle of winter because the atmosphere can hold more moisture. It's about, you know, 4 percent for every degree Fahrenheit warmer. The atmosphere can hold more moisture as long as there's a supply of moisture from somewhere. This year, the whole of the jet stream has been pushed well to the North. And in North America, it's just been remarkably mild throughout the, you know, all of this winter and especially in March.

CONAN: And we saw some - to say the temperatures were unseasonable seems an understatement.

TRENBERTH: Yes. Well, you know, the official statistics, I think, in March said that there were, what, 7,755 U.S. temperature records broken in March, you know, and that's absolutely remarkable. It's certainly an indication that we're not experiencing normal weather anymore. It's not the kind of thing one expects under an unchanging climate, and, therefore, it indicates that the climate is indeed changing. So this has got a global warming element to it.

CONAN: So as the climate warms, what kinds of changes can we expect? And again, a lot of variation but as a rule.

TRENBERTH: Yes. And so the main memory of the warming occurs in the oceans. The oceans have warmed up. And, in general, they're about a degree Fahrenheit warmer than they were, say, back in the early 1970s. And as a result, the air is warmer and moister over the oceans. Over land, the land can dry out a little quicker. This is especially relevant when we have droughts such as we had last year in Texas. And the drought sets in a little quicker. It is more intense, longer-lasting and then sets the stage for wildfires.

And so those are the cumulative effects that are really important. And the key thing then is that all of the weather that is occurring, all of the storms are occurring in an environment that is simply different than it used to be. It's generally a warmer weather environment, and it means that the rains tend to be heavier. We saw a special case of that last year with Hurricane Irene dumping all kinds of wet weather along the East Coast and flooding many areas. And then the droughts are tending to be longer and hasher with increased risk of wildfire. And we've seen that in the U.S. over the last year as well.

CONAN: One degree warmer in the oceans, it doesn't sound like a lot.

TRENBERTH: It doesn't sound like a lot. It corresponds to about a 4 percent increase in moisture on average, which can - or four or 5 percent. And it can easily be five to 10 percent when it gets caught up in storms and provides the extra buoyancy in storms, and so the storms become invigorated. And this applies to tropical storms. It can apply to individual thunderstorms.

And the key thing then is that we always have weather, some of which is going in the same direction as the climate change. And when we approach those, what used to be limits before, now we're going through them and we're breaking records. And you know, this is sometimes the straw that breaks the camel's back kind of syndrome. You cross the threshold and suddenly you're in a completely different territory, and some bad things can happen as a consequence.

CONAN: Well, we'll get some people on the line who have some questions for you, but I wanted to ask latitude. Are the same effects - are the effects roughly the same at the latitude of New York or San Francisco as they are much further north in the Arctic?

TRENBERTH: Well, the Arctic is a special case, of course, because of the Arctic sea ice, and there's been a tremendous loss of Arctic sea ice, which peaks in the summertime, about a 40 percent decrease since the 1970s. And that's absolutely remarkable. And that has influences into the fall and into the winter and is probably affecting some of the weather patterns. And it certainly affects snow in the surrounding areas because it's sort of like the lake effect that occurs in Buffalo, New York, prior to the Great Lakes freezing over. You get a lot of snow in surrounding areas because the winds just pick up the moisture off of the open ocean and dump it down in the form of snow.

And so this can have an influence on the weather patterns. And it may be one of the things which has helped to weaken the jet stream a little bit and therefore enabled the weather patterns to get stuck a little more. And therefore we get more persistent warm periods and sometimes even cold periods. And so, you know, in late January, early February, there was a very cold outbreak in Europe last year - this year, sorry.

CONAN: Yeah, earlier this year and...

TRENBERTH: Yes.

CONAN: ...just to say that there was that extraordinary storm in Alaska off the Bering Sea, but it's not the storm that was all that extraordinary. It's just that there was so much less ice that was protecting the coastal villages that the water was washing directly ashore.

TRENBERTH: Yes. And so the sea ice is changing. Alaska has actually been fairly cool this year though, while the rest of the U.S. has been quite warm this past winter.

CONAN: Let's get some callers in on the conversation. We're speaking with Kevin Trenberth, a distinguished senior scientist at the National Center for Atmospheric Research and chairman of Global Energy and Water Cycle Experiment at the World Climate Research Program. We'll start with Isaac, and Isaac is on the line with us from Camden in South Carolina.

ISAAC: Hey. How are you guys? Good to hear about this conversation. I'm a geologist in Camden and lived in South Carolina mostly my entire life. I moved here after Hugo. And the hurricane season seems to kind of always miss the Carolina coast and Northern Georgia. What's the control there? Is it the Gulf current? Is it the jet stream? And is it something that, let's say, climate change would, you know, maybe disrupt that control and we'd get hit by back-to-back Hugos? Is that something that we could maybe, you know, preparing for?

CONAN: Or at least worry about. Kevin Trenberth?

TRENBERTH: Yes. So with hurricanes, you know, the main control over where an individual hurricane goes happens to be the weather situation, just where the low and high pressure systems are, where the jet stream is. And so there's a fair bit of a crapshoot as to exactly where the track is. The track does change a bit seasonally, whether it's an early season, mid season or late season. There tend to be distinctive patterns. And it does change a bit with influences like El Nino. In fact, El Nino tends to suppress activity in the Atlantic, and there are good prospects for that this year, a less active hurricane season this year. With global warming, we do expect that there will be more intense storms, but exactly where they go is, as I say, a bit of a crapshoot.

ISAAC: OK. Well, actually, I have one more question. This is a little bit more abstract. One of my professors back in undergrad told me that if there were a massive melting of continental ice, it would increase the volume of water in the ocean, thus increasing the weight of water on the crust and possibly leading to a massive scale volcanism that would lead to nuclear winter. Is that a possible outcome in global warming?

TRENBERTH: Well, there's a bunch of steps there.

(SOUNDBITE OF LAUGHTER)

TRENBERTH: Certainly melting of ice on land is happening. You know, Greenland is melting a little bit. The glaciers are melting. But, of course, it doesn't change the overall weight on the planet. It's redistributing it. And so it goes into the ocean (technical difficulties) rising at a rate of just over three millimeters per year, 2.2 inches since 1992, which is when we've had global measurements of sea level from satellites that are looking down from space with altimeters that can actually measure the sea level to millimeter accuracy. And so sea level is definitely going up, and it does, as you say, change the distribution of weight on the crust, which begins to adjust. And it may indeed have some geological consequences in that regard.

ISAAC: Let's hope (unintelligible) the sea is on our side.

CONAN: Thanks very much, Isaac.

ISAAC: Thank you.

CONAN: We're talking with Kevin Trenberth, distinguished senior scientist at the National Center for Atmospheric Research. You're listening to TALK OF THE NATION from NPR News. And you mentioned the one degree increase in temperature in the oceans. You mentioned that rise in the water level as a result of ice melt. Is it accelerating? Can people tell that?

TRENBERTH: Well, it has accelerated from certainly the 20th century and especially the early part of the 20th century somewhat. Sea level went up as best we can tell through, you know, they we're just looking at individual islands and a number of coastal stations where there's long records. And it's also a little complicated by the fact that the land can move around because of, you know tectonic activities. But, you know, the best estimate overall is that sea level went up about six inches in the 20th century. And, you know, we've gone up about two inches, a bit over two and a quarter inches since 1992.

So we're going up - sea level is going up at the rate at over - a bit over a foot per century, but there are a number of expectations that this could easily double and - or even triple, you know? It could be as much as three feet by 2100. Although I think that's probably pushing it a little bit, but certainly the rate is not slowing down.

CONAN: Let's go next to Gary. Gary with us from Cincinnati.

GARY: Yes. Thank you. My question is, as we rotate on our axis, I understand that there's an adjustment that's taking place there. Earthquakes can affect that. And then the other thing we have that's currently going on is the solar flares of the sun, and that has an effect on the electromagnetic field. And so I was wondering how the two of those affect our weather changes.

TRENBERTH: All right. Yes, the winds can change over the surface of the oceans and also in blowing up against mountains and things like that, which causes some stresses on the surface of the Earth. Usually it's pretty resilient to that. The biggest changes that we observe are associated with the El Nino phenomenon, and they actually cause a change in the length of day, which is measurable. It's a very tiny amount. It's about a nanosecond or something like that. But it is actually measurable. So the whole rate at which the earth is rotating can change a little bit because of the - because of those kinds of effects. But the effects on weather are probably miniscule.

The sun activity, it's not just the flares and the electromagnetic effects. It's also the effects on the radiation and especially in the ultraviolet, which can affect the ozone layer. And so we went through a period up until about one or two years ago, say, where it was very quiet. The sun was very quiet, and people were speculating that maybe the sun was not going to be as active in the future, but certainly it's been active in the last year or so, back to what we've seen before. And in the quiet phase, it does cool - cause a very slight cooling of the climate. So it probably offset a little bit of the global warming that we otherwise would've seen. The estimate is that, you know, maybe 10 or 15 percent.

CONAN: Wow.

TRENBERTH: That may be why we haven't seen as much - quite as much warming as we might have expected, but with the sun, you know, more active now, it certainly sets the stage that the global warming effects are not being offset, and so they begin to show their face a little more.

CONAN: Gary, thanks very much for the call. And, Kevin Trenberth, can you tell us what one indicator do you watch most carefully?

TRENBERTH: Well, it depends on what for, but if you wanted to watch one indicator for global warming, I would say probably the global sea level now is one of the better ones because it integrates two effects. It integrates the warming of the oceans, which are then expanding, and it also throws in the component relating to the melting of land ice. And it's much less subject to weather effects. And so when we're looking at temperatures, you know, one of the main indicators that many people follow is the estimates of the global mean temperature. But that can actually vary quite a bit with weather, whether the warm spots are on land versus over the ocean, and it can also vary a bit with El Nino. When we get an El Nino following an El Nino is when we tend to have the highest global mean temperatures. And during La Nina conditions, when its' cold out in the tropical Pacific, you know, over a region which is bigger than the United States, then the whole of the - the global mean temperature tends to be a bit lower and it can have influences like that in the Pacific Rim countries. So it's much more subject to some of these so-called natural variability effects.

CONAN: So watch for the level of the oceans. Thanks very much.

GARY: Yes.

CONAN: Kevin Trenberth of the National Center for Atmospheric Research joined us from his office in Boulder, Colorado. Tomorrow, TALK OF THE NATION: SCIENCE FRIDAY, a look at biodiversity in cities. We'll be back on Monday to talk about one of the most loved and hated teams in sports: the New York Yankees. Damn Yankees. I'm Neal Conan. This is NPR News.

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